George Chierici

Primate experiments on oral respiration

Oral respiration associated with obstruction of the nasal airway is a common finding among patients seeking orthodontic treatment. The primate experiments reported here are part of a series designed to test some of the current hypotheses regarding the relationship between mouth breathing and dental malocclusions, that is, between deviations in orofacial muscle recruitment and jaw morphogenesis. Mouth-breathing was developed in the animals of this experiment by obstruction of the nasal passages with silicon nose plugs. The experiments showed that the monkeys adapted to nasal obstruction in different ways. In general, the experimental animals maintained an open mouth. Some increased the oral airway rhythmically, while others maintained the mandible in a lower position with or without protruding the tongue. All experimental animals gradually acquired a facial appearance and dental occlusion different from those of the control animals. From these and the previously reported primate experiments in this laboratory, it can be deduced that orthodontic appliances in general affect the morphology of the orofacial structure in two ways: by direct force and by sensory stimulation. (1) The appliance exerts a direct physical force which alters the strain distribution in the bone and elicits bone remodeling and tooth movement. (2) The presence of the appliance initiates the sensory input which triggers a neuromuscular response. This change in neuromuscular activity, in turn, affects both muscle development and bone remodeling. The fixed orthodontic appliance may work mainly on the first principle. Certain removable appliances may have a significant effect based on the second principle.

Primate experiments on oral sensation and dental malocclusions

1 n contrast to the vast array of metric observations on human skulls during the growth period, there is a paucity of information regarding the physiologic correlates: the factors controlling growth at the mandibular condyles, the pressure and tension systems determining bone apposition and modeling on the surfaces of the jaws, and the functional systems regulating tonus in the musculature which positions the mandible relative to the maxilla. Pilot studies1 have demonstrated that the rhesus monkey can serve as an experimental model in research on the etiology of dental malocclusions. With the use of this model, it has been possible to test certain hypotheses on the interrelationships of muscle function, skeletal form, and dental malocclusion by inductive inference rather than by surveillance. Alternative hypotheses are formulated and subjected to testing. Recent experiments have been designed to test certain theories regarding functional factors which affect the suspension of the mandible as well as the position of the teeth. The relationship of factors which influence the establishment of dental occlusion is illustrated in Fig. 1, in which A and B represent the maxilla and the mandible; C, the tongue; D and E, the muscles suspending the jaw; and P and G, the erupting or extruding teeth. The model illustrates the possible effects of changes in the relative tonus of the muscle groups (D and E) upon the distance between the jaws (A and B) as well as the space available for the tongue (C) and the extruding teeth (P and G). The question becomes: Can significant changes in muscle tonus occur in response to common environmental stimuli ? Obviously, strong orthodontic forces, muscle exercises, and selective destruction of neuromuscular components

Morphologic response to changes in neuromuscular patterns experimentally induced by altered modes of respiration

The present experiment was designed to test whether specific recordable changes in the neuromuscular system could be associated with specific alterations in soft- and hard-tissue morphology in the craniofacial region. The effect of experimentally induced neuromuscular changes on the craniofacial skeleton and dentition of eight rhesus monkeys was studied. The neuromuscular changes were triggered by complete nasal airway obstruction and the need for an oral airway. Alterations were also triggered 2 years later by removal of the obstruction and the return to nasal breathing. Changes in neuromuscular recruitment patterns resulted in changed function and posture of the mandible, tongue, and upper lip. There was considerable variation among the animals. Statistically significant morphologic effects of the induced changes were documented in several of the measured variables after the 2-year experimental period. The anterior face height increased more in the experimental animals than in the control animals; the occlusal and mandibular plane angles measured to the sella-nasion line increased; and anterior crossbites and malposition of teeth occurred. During the postexperimental period some of these changes were reversed. Alterations in soft-tissue morphology were also observed during both experimental periods. There was considerable variation in morphologic response among the animals. It was concluded that the marked individual variations in skeletal morphology and dentition resulting from the procedures were due to the variation in nature and degree of neuromuscular and soft-tissue adaptations in response to the altered function. The recorded neuromuscular recruitment patterns could not be directly related to specific changes in morphology.

Experiments on the development of dental malocclusions

F urther clarification of the development of dental malocclusions is needed before prevention and treatment can be systematic rather than symptomatic. Considerable insight has been gained over the past 75 years, the period of organized orthodontics in dentistry, but many questions still remain unanswered. Among these are some questions which relate to problems apparently blocking progress in orthodontics. For example, which factors determine (1) the forward growth of the maxilla, (2) the growth of the mandible, (3) the suspension level of the mandible relative to the maxilla, and (4) the location of the occlusal plane between the jaws? A review of the orthodontic literature reveals that the most common approach among investigators has been first to classify the malocclusions on the basis of appearance and then to focus attention on the etiologic factors. Observations on changing appearance during growth do not contain the information required to answer the questions mentioned above. Such data can serve only as a basis for hypotheses which must be tested experimentally. Focusing on the etiologic factors has proved advantageous in microbiology, pharmacology, and related fields in which the main interest is centered on the qualities of the causative factor-the microorganism or drug, for example. In orthodontics, this approach is less fruitful because the lists of causes and the spectra of effects obscure the mechanism of actions which is, after all, the subject of greatest interest when it comes to prevention and treatment. The mechanism of action is determined by the response of the orofacial structures to outside stimuli. It may be more advantageous in orthodontics to focus on the response mechanisms and merely group the stimuli according to the responses they elicit. Only certain common denominators among the etiologic factors are of

Experimentally induced neuromuscular changes during and after nasal airway obstruction

Neuromuscular changes were studied by electromyography in rhesus monkeys which adapted to nasal obstruction for 2 years and then in the succeeding year recovered to nasal respiration. Obstructing the nasal passage with silicone plugs induced specific behavioral responses which remained for the duration of nasal obstruction and were lost within 8 days after removal of the plugs. Animals demonstrated individual variations, but more than 80% consistently maintained a lower mandibular posture for the entire 2-year period. Rhythmic mandibular, tongue, and upper lip movements were evident in fewer than 60% of the animals. Certain craniofacial and tongue muscles (the genioglossus, dorsal tongue fibers, digastric, geniohyoid, dilator naris, and vertically oriented fibers of the superior orbicularis oris, that is, lip-elevator fibers) were recruited rhythmically and remained rhythmically active throughout the entire 2-year period of nasal obstruction. This rhythmic activity ceased within 1 week after removal of the nose plugs. A tonic or consistent discharge was also induced in the genioglossus, dorsal tongue fibers, the geniohyoid, superior orbicularis oris, and lip-elevator fibers over the entire 2 years of nasal obstruction. Not all muscles lost their tonic discharge after removal of the nasal plugs. The genioglossus, geniohyoid, inferior orbicularis oris, and lip-elevator fibers discharged tonically during the recovery period. These data suggest that nasal obstruction can induce neuromuscular changes which extend beyond the period of obstruction and remain after the original stimulus for neuromuscular change has been removed.

Sequential neuromuscular changes in rhesus monkeys during the initial adaptation to oral respiration

Experimental induction of oral respiration in primates altered the neuromuscular use of specific craniofacial muscles. Obstruction of the nasal passage in the rhesus monkey induced changes in the electromyographic discharge (EMG) of both mandibular and facial muscles during the first 6 months of adaptation. Eighteen craniofacial muscles were studied with regard to their type of neuromuscular pattern. The EMG discharge was analyzed in terms of whether it had a rhythmic discharge or a continuous recruitment of motor units. The results of the investigation revealed that certain muscles in the control monkey using nasal respiration could be rhythmically or continuously active, but no significant trend was apparent with either pattern over time. In contrast, a significant number of muscles became rhythmically active within the first month of adaptation to oral respiration in the experimental animals. The rhythmic pattern was evident in key muscles that actively depressed the mandible, protruded the tongue, altered the shape of the tongue, and raised the upper lip. Continuous activity was induced in the first month within the suprahyoid region and tongue but later, by the fifth month, in specific lip and elevator muscles. These results suggested that the neuromuscular system adapted immediately to nasal obstruction but would vary as to (1) which muscles would be important in the initial adaptation, (2) the mode of adaptation, and (3) the time when a particular pattern first began to be used.

Clinical speech considerations in prosthodontics: Perspectives of the prosthodontist and speech pathologist

Abstract Speech is a dynamic process with which prosthodontists are involved. Should a denture patient manifest difficulty in speech, it is crucial to differentiate defects which are denture-related from those produced by other coexistent conditions. Seven dimensions basic to speech are discussed: (1) respiration, (2) phonation, (3) resonance, (4) speech articulation, (5) audition, (6) neurologic integration, and (7) emotional behavior. Clinical aspects of these factors as they relate to prosthodontics are described. For speech purposes, static, positional concepts of incisor relationships and denture contours should not be emphasized at the expense of dynamic considerations. Each patients condition should be evaluated to assure that the denture can provide an optimal environment for the rapid, coordinated muscle movements requisite for acceptable speech.

Electromyographic analysis of the functional components of the lateral pterygoid muscle in the rhesus monkey (Macaca mulatta)

Electromyographic recordings (EMG) obtained from intramuscular electrodes inserted in the lateral pterygoid muscle (lat. pt.) of 29 juvenile and adolescent monkeys were assessed in relation to the recorded activity of three jaw elevating muscles, temporalis, masseter and medial pterygoid, and a jaw muscle depressor, the geniohyoid. The lat. pt. was recruited in one of three patterns: (1) only during mandibular depression, (2) during both depression and elevation, or (3) only during elevation of the jaw. The discharge of fibres in the lat. pt. during closing movements occurred bilaterally in 9 of 21 recordings. Simultaneous recordings from the anterior temporalis, medial pterygoid and the ipsilateral lat. pt. indicated completely different discharge patterns of the three muscles. In 9 recording sessions, two pairs of electrodes were placed in the same lat. pt. The discharge patterns from the two pairs were different in 5 of the recordings. In one region, the fibres were active only during depression, whereas in the other region fibres discharged only during elevation of the mandible. The data support the concept of two groups of fibres with separate functions within the lat. pt. and suggest that adaptation to oral respiration utilizes the two groups in distinctive patterns.

Adaptation in the Function of Pharyngeal Constrictor Muscles

The palatopharyngeus and pharyngeal constrictor muscles were studied by electromyography (EMG) and by direct observation with a flexible fiberoptic scope in the anesthetized as well as in the alert rhesus monkey. The muscles were monitored to determine the change in their discharge with nasal obstruction, head posture, head extension, and swallowing. The results indicated that certain regions of the middle and inferior pharyngeal constrictors never discharged during deglutition. Extending the head could induce a tonic discharge in fibers of the middle pharyngeal constrictor for the duration of head extension. Placement of water in the hypopharynx not only induced a sustained laryngospasm but also a tonic discharge in the select fibers of the superior and middle pharyngeal constrictors. Changing from a supine to an upright posture, or obstructing the nasal cavity, could induce a rhythmic discharge. These studies indicate that there are functional components of fibers within each of the anatomically recognized pharyngeal constrictors.

Experimental study of muscle reattachment following surgical detachment

The process of muscle reattachment was studied in rhesus monkeys using electromyographic and histologic techniques. The attachments of the temporal muscle were exposed on both sides and the muscle was detached from its origin on the right side in ten rhesus monkeys. EMG activity was recorded by fine wires placed intramuscularly within the anterior, middle, and posterior parts of the muscles before and after detachment. The control EMG activity recorded in the attached muscle showed a wide range of values, and there were no clear trends. Following detachment the range of values was also wide, and the differences in activity between the detached muscle and the contralateral attached muscle were not outside the range of difference normally observed between right and left sides. EMG activity was greater before detachment in 31.2% of the recordings and was greater after detachment in 29.4% of the recordings. Biopsies of bone and muscle were taken from normal attachment sites on the left sides and also from the right sides following detachment at one, two, three, four, six, and eight weeks and examined by light microscopy. New bone spicules were seen developing from the surface of the bone after two weeks that were oriented in the direction of the muscle fibers. The reattachment process is one in which new bone is formed on the surface of the bone and develops toward the end of the muscle to envelop the reorganizing tendon. Bone formation occurring on the surface of bones at the ends of muscles is not dependent on tension or viscoelastic properties of the muscle.